1. Field
Embodiments relate to methods and apparatus for improved power distribution within a folded flex package and, in particular, to substrate design to enable even power distribution across a fold region.
2. Description of the Related Art
Electronic devices such as cellular telephones and notebook computers typically contain a number of integrated circuit (IC) packages mounted to a printed circuit board (PCB). IC packages typically include a single IC die (or chip) on a substrate or leadframe. The die and substrate are encapsulated in a material, such as plastic. The encapsulated packages are then mounted to another substrate, such as a PCB.
Multichip modules (MCM) are IC packages that can contain two or more integrated circuits. The size of the electronic device that uses MCMs can be reduced because MCMs typically have a number of individual IC dice mounted within a single package in a laterally adjacent manner. The outer dimensions of all the individual elements, however, limit the minimum footprint of a MCM.
Moreover, MCM substrates are typically constructed from ceramic, silicon, metal or printed circuit board materials that are relatively expensive to produce. Considerable effort has been expended to provide an electronic package that has a minimal footprint and volume and that can be assembled with conventional plastic injection molding techniques without adding expensive interconnecting substrate components.
FIG. 1 illustrates a design that attempts to decrease the footprint and volume of the IC package. Processing device 100 is a standard stacked chip scale package (SCSP) configuration. Processing device (e.g., IC package) 100 includes stacked IC dice, which include logic die 101 and memory die 102. The SCSPs are formed by stacking several sets of IC dice on a substrate, wire bonding, encapsulating the IC dice, and then slicing the substrate and encapsulant to separate each SCSP.
Processing device 100 includes conductive traces 114 on the top surface of the substrate. Vias are conductive interconnects that extend through the substrate to electrically connect traces 114 to conductive pads on the bottom surface of the substrate. One example of a substrate is a printed circuit board (PCB). Other examples of materials for the substrate are: flame retardant 4 (FR4), bismaleimide-triazine (BT), tape automated bonding (TAB) tape material, ceramic, silicon on sapphire (SOS), or a multi-layered substrate such as an organic land grid array (OLGA).
The SCSP processing device 100 shown in FIG. 1 is connected to a circuit board (not shown) by solder balls 118, which are placed on pads on the bottom surface of the substrate. Other types of IC packages may include leads that extend laterally with respect to the dice within the package for connection to an external circuit board. An adhesive, such as epoxy, is used to mount die 101 and die 102. After first die 101 and second die 102 are mounted, they are wire bonded to the substrate 105. First die 101 has bond pads on its top surface near its edges, and second die 102 has bond pads on its top surface near its edges. Bond wires connect the bond pads of first die 101 and second die 102 to substrate 105. The bond pads are connected by bond wires to bond fingers 240 (see FIG. 2).
FIG. 2 illustrates a layout for a folded flex substrate, such as the SCSP processing device 100 illustrated in FIG. 1. Folded flex substrate 200 includes a first area (portion) 210 where a memory die is to be placed. A second area (portion) 220 is where a logic die is to be placed. Fold region 230 is where the folded flex substrate is to be folded over. Folded flex substrate 200 also includes bond-fingers 240. Collector and source routing 250 are routed across the fold region.
For flex SCSP (FSCSP) package stack configurations, as illustrated in FIG. 3, power delivery through the fold region is very challenging. This stack configuration has a bottom portion as illustrated in FIGS. 1-2. The signal trace length is effectively doubled in a FSCSP package, which directly impacts power delivery inductive resistance (IR) drop. This in turn adversely impacts product performance. Device 300 in FIG. 3 includes first device 310 and second device 320 arranged in a stack configuration. Balls 310 are coupled to pads on a substrate. Vias extend through the substrate to electrically connect traces 314 to conductive pads on the bottom surface of the substrate.
The above described devices having current folded flex substrate design are prone to performance issues due to length of traces from a bottom solder ball in a ball grid array (BGA) to a top package interface. This results in power delivery horizontal IR drop between the respective power and ground buses.